A wireless communication system has a method for switching a transmission mode in accordance with a reception quality as a method for realizing the data transmission with a high speed and a high quality. The transmission mode to be switched is different depending on the reception quality, however, as a parameter for differentiating the content of the transmission mode, there are provided an encoding rate of a modulating system and an error correction code. For example, at a transmission side, an encoding rate k/n of an error correction code that a redundant bit of (n-k) bits is added to information bit of k bits is selected in accordance with the reception quality and further, a modification system such as QPSK, 16QAM, and 64QAM or the like capable of transmitting two bits, four bits, and six bits for one modulation, respectively is selected in accordance with the reception quality.
The more the encoding rate and the number of the modulation bits capable of being transmitted are, the higher the highest data transmission rate is, however, the reception quality (for example, SNR: a rate between a signal power and a noise power) to satisfy a target communication quality (indicated by a block error rate, a bit error rate, and a throughput or the like) becomes also higher. In the wireless communication system, the reception quality is varied depending on a propagation path among wireless apparatuses and interference from other wireless apparatuses or the like. Therefore, if the data is transmitted in the optimum mode that the data transmission rate becomes the highest in the transmission modes due to the modulating system and the encoding rate (hereinafter, abbreviated as a modulating/encoding mode) that can satisfy the target communication quality in accordance with the reception quality, it is possible to maximize the throughput of the system.
The structure of a conventional wireless apparatus for switching the above-described modulating/encoding mode is shown in FIG. 1 and FIG. 2. In a wireless apparatus 101 shown in FIG. 1, a signal separating unit 105 separates a control signal from a reception signal which is transmitted from an antenna 102 via a duplexer 103 and a receiver 104. A control signal demodulating unit 106 demodulates control signal and extracts reception quality information. A modulating/encoding mode selecting unit 108 compares the reception quality with a switching threshold of the modulating/encoding mode to select the modulating/encoding mode in accordance with the reception quality.
A modulating/encoding unit 109 carries out the error correction encoding and the modulation for a data signal in accordance with the selected modulating/encoding mode. A signal multiplex unit 110 multiplexes a data signal which is the output of the modulating/encoding unit 109 together with a control signal having modulating/encoding mode information and a pilot signal. Then, the output of the signal multiplex unit 110 will be transmitted from the antenna 102 to a wireless apparatus 121 shown in FIG. 2 via a transmitter 111 and a duplexer 103.
In the wireless apparatus 121 shown in FIG. 2, a signal separating unit 125 separates the pilot signal from a reception signal which is transmitted from an antenna 122 via a duplexer 123 and a receiver 124. A reception quality estimating unit 129 estimates SNR of the reception signal from the pilot signal. A signal multiplex unit 130 multiplexes the control signal having receipt quality information on a data signal. Then, the output of the signal multiplex unit 130 will be transmitted from the antenna 122 to the wireless apparatus 101 of FIG. 1 via a transmitter 131 and a duplexer 123.
As shown in FIG. 6A, a switching threshold value of the modulating/encoding mode is set at a fixed value in advance so as to indicate a range of the reception quality satisfying the target communication quality. According to an example of FIG. 6A, three modulating/encoding modes (hereinafter, abbreviated as MCS, here, it is defined that MCS#1 is QPSK, R=½, MCS#2=QPSK, R=¾, MCS#3=16 QAM, and R=½, respectively) are switched by a first threshold value T1 and a second threshold T2. In other words, when the reception quality <T1, MCS#1 is selected, when T1≦the reception quality <T2, MCS#2 is selected, and when the reception quality≧T2, MCS#3 is selected. As a result, it can be said that the better the reception quality is, the more the higher-speed data transmission can be carried out.
However, even if the reception quality is the same, when the propagation environment is different, the optimum modulating/encoding mode becomes different. A determining factor of this propagation includes a multipath environment (the number of path and delay dispersion) and the highest Doppler frequency (mobile speed) or the like, The fact that the optimum modulating/encoding mode is different even when the reception quality is the same leads to that the threshold value of the reception quality for selecting the optimum modulating/encoding mode is changed when the propagation environment is changed. The more the propagation environment is changed, the more the threshold value for selecting the modulating/encoding mode is changed. Therefore, in the case of the method of selecting the modulating/encoding mode comparing the reception quality with the fixed threshold value, it is difficult to have the optimum threshold value.
With reference to a JP-A-2003-37554 as a method for solving the above-described problem, there is provided a method for variably controlling the threshold value on the basis of with or without of the reception error in units of information blocks. According to the above-described wireless apparatus 121, a demodulating/decoding unit 127 demodulates a data signal in accordance with the modulating/encoding mode laid on the control signal. A block error detecting unit 128 detects with or without of the block error from a demodulation result of the data signal. This detection result of the block error will be transmitted to the wireless apparatus 101 being laid on the control signal.
According to the wireless apparatus 101, the signal separating unit 105 separates the control signal from the reception signal. The control signal demodulating unit 106 demodulates the control signal and extracts a detection result of a block error. The threshold value control unit 107 variably controls the threshold value of the reception quality on the basis of the block error which is noticed from this reception side.
As the threshold value control unit 107, for example, there are provided methods shown in FIG. 6B and FIG. 6C. These drawings show the control methods in the case that the modulating/encoding mode of MC#2 has been used right now, respectively. According to the method of FIG. 6B, when the reception of the information block succeeds, the threshold value T1 and the threshold value T2 are decreased by a predetermined control amount Δ down dB, and when the reception of the information block fails, the threshold value T1 and the threshold value T2 are increased by the predetermined control amount Δ up dB. In the method shown in FIG. 6C, when the reception of the information block succeeds, only the threshold value T2 is decreased by the predetermined control amount Δ down dB, and when the reception of the information block fails, only the threshold value T1 is increased by the predetermined control amount Δ up dB.
In this case, assuming that a target block error rate is 1/N, Δ down and Δ up are set in the relation of Δ up=(N−1)×Δ down. Under this control, when the average block error rate of each modulating/encoding mode coincides with the target block error rate, the volumes of Δ down and Δ up balance and the threshold values stop (converge) here. Thereby, even if the propagation environment is changed, the threshold value to select the modulating/encoding mode can be adaptively set so as to keep the regular communication property.
According to the above-described conventional art, it is possible to optimize the threshold value of the reception quality to select the modulating/encoding mode in accordance with change of the propagation environment to some extent; however, it seems that the deviation from the optimum value is generated when the change of the propagation environment is large. FIG. 7 shows a throughput property with respect to the reception quality in a propagation environment. In FIG. 7, a solid line shows the throughput property in the good propagation environment and a broken line shows the throughput property in the bad propagation environment, respectively.
Comparing the both, the threshold values T1 and T2 to select the modulating/encoding mode are changed into T1′ and T2′, and the volumes of the throughput, namely, P1 and P2 are changed into P1′ and P2′, respectively. The throughput of each modulating/encoding mode is represented by the maximum data transmission rate×(1−a block error rate) which can be sent by its modulating/encoding mode, so that lowering of throughput means that the average block error rate of each modulating/encoding mode is increased. Accordingly, in order to control the switching threshold value of the modulating/encoding mode optimally, it is necessary to change the target block error rate in accordance with the propagation environment.
An object of the present invention is to provide a wireless apparatus, a wireless communication system, and a transmission mode selecting method, whereby a transmission mode in accordance with a reception quality and a propagation environment can be optimally selected.